MR, T. W. BRIDGE ON THE OSTEOLOGY OE POLYODON EOLIUM. 
715 
If this rod, instead of completely disappearing as it. does in the Frog and the Shark, 
were to increase in size and persist, it would occupy a position analogous in every 
way to that of the fibrous band in Polyodon. 
It may be remarked that the “ sub -notochordal rod” of these Vertebrata occupies a 
position in relation to the aorta which is not altogether without analogy to the position 
of the “ endostyle ” of the Ascidians in relation to the blood sinus which in these 
animals lies on the haemal side of the branchial sac.* 
Skeleton of the Fins. 
The structure of the pectoral fins has been described and figured by Gegenbaur.I 
The dorsal and anal fins are each supported by two series of well-ossified, shaft-like 
interspinous bones, each bone supporting at least two fin rays. There is also a third 
series of interspinous cartilages formed by a number of small cubical pieces of cartilage 
which occupy the interspaces between the distal ends of the bones of the second series. 
A similar peripheral fringe of small cartilages, and similarly arranged, is found in the 
pectoral fins, and also in the ventral portion of the caudal fin attached to the free ends 
of the inferior caudal arches. In the Selachii a similar peripheral fringe of small 
cartilages is to be found in the mesial, and in some cases in the paired fins also. None 
of the interspinous bones or cartilages exhibited any of that tendency to coalescence 
which is so marked in the mesial fins of the Selachii. On the other hand, the evident 
formation of the ventral fins by the coalescence of a series of originally distinct carti- 
laginous rays is clearly indicative of a more primitive condition of these structures 
than can be found in any other living Vertebrate animal. J 
SUMMARY AND CONCLUSION. 
In the foregoing description of the skeletal structures of Polyodon I have compared 
the latter with Acipenser, in so far as the special features under discussion were con- 
cerned, in order to demonstrate the close relationship that exists between the two 
forms in almost every detail of their anatomy. It will now be necessary to compare 
the two genera as to the points in which they differ from one another, and also 
to compare them with the remaining recent Ganoidei on the one hand, and with the 
Elasmobranchii and Amphibia on the other. In order to do this, it will be advanta- 
geous briefly to recapitulate the more important anatomical facts with which such a 
comparison is concerned, and which have been given in detail in the earlier part of 
this paper. 
In the following paragraphs are enumerated those features in the osteology of 
Polyodon and Acipenser which are of greatest importance in relation to more general 
conclusions. 
* Vide also Dr. Hugo Eisig’s paper oil “ Nebendarm cl. Capitelliclse ” in the ‘ Zoologischer Anzeiger,’ 
No. 7, 1878. 
t Op. cit. Plate 8. 
f Thacheb, loc. cit. 
